\documentclass{article}      %%% LaTeX2e
 \hyphenation{arch-bishop dis-cre-tionary}
 \textwidth 62mm
\begin{document}
  In this paragraph the discretionary command (which is not
  di\discretionary{f-}{fi}{ffi}cult to understand) is used.
  If the german word `Zu\discretionary{k-}{k}{ck}er' might
  possibly be hyphenated, you must use the discretionary command.
\end{document}
